There are many industrial uses for the machine classification of solids but the preponderant use thereof normally lies in the treatment of raw materials, such as mineral ores or other similar substances. Wet classification is defined as the art of separating the solid particles in a mixture of solids and liquid into fractions according to particle size or density by methods other than screening. Generally wet classifiers operate upon the difference in settling rate between coarse and fine or heavy and light gravity particles in the liquid medium so the coarse particles have a relatively faster settling velocity than fine particles of the same specific gravity and as heavy gravity particles have a relatively faster settling rate than light gravity particles of identical size. The rate of settling in some wet classifiers is also controlled by some extent by agitating the liquid medium, thus providing hindered settling.
In addition to use in separating the metallic ores, wet classification methods have also been applied in the preparation of coal. Coal preparation has become of greater importance in recent years as a result of an increase in mechanization in mining operations, which together with the growth of strip mining and depletion of the better quality coal seams in many coal fields has meant that run of mine coal contains increasingly larger quantities of foreign substances. These foreign substances tend in most cases to significantly increase the sulfur content, which of course is undesirable from an air pollution standpoint, thereby reducing the value of the coal obtained or rendering same unmarketable.
Wet classifiers in the coal industry have assumed a myriad of forms, ususally dependent upon the size of coal involved. The most widely employed classification methods are, however, jig and trough washing. An example of a trough type washer is disclosed in in the U.S. Pat. No. 3,739,911, issued June 19, 1973. The pool-less auger separator described therein uses an open ended inclined trough having an auger in the bottom thereof. Coal and water are introduced at an intermediate point along the trough. The turbulent or agitated water flowing down the trough is regulated so that the coal is transported down the trough but the foreign substances, such as rocks, shale, fine clay and ash, having a specific gravity higher than the coal, settle to the trough bottom and are transported to the upper end of the trough by the auger thus separating the materials. In actual operations the device described has been found adequate for the removal of coarse rocks but the finer particles largely pass unhindered through the trough with the coal.
It is therefore an object of this invention to provide a means of separation which will efficiently and economically remove the fine heavy particles as well as the coarser heavy particles from the intermediate specific gravity materials with a liquid having a specific gravity less than any of the solids.